1. Field of the Invention
The present invention relates to a managed pressure and/or temperature drilling system and method.
2. Description of the Related Art
Natural gas hydrates are individual molecules of natural gas, such as methane, ethane, propane, or isobutene, that are entrapped in a cage structure composed of ice molecules. The hydrates are solid crystals with an “ice like” appearance. Gas hydrates exist in environments that are either high pressure or low temperature or both and have been found in subsea ocean floor deposits and in subsurface reservoirs both on and offshore. The amount of “in place” gas hydrates in the U.S is estimated at 2,000 trillion cubic feet which is equivalent to the produced or known natural gas deposits. For a more in depth analysis of the vast potential of gas hydrates, see SPE/IADC 91560 entitled “MPD—Uniquely Applicable to Methane Hydrate Drilling” by Don Hannegan, et. al (2004).
FIG. 1 illustrates simplified disassociation boundaries for various gas hydrates. The curves may vary depending on the amount of gas trapped in an amount of hydrate. To the left of the curves, formed gas hydrates are in a solid phase. To the right of the curves, the hydrates will disassociate into gas (and water and/or ice). Note also, that a disassociation curve and a formation curve (not shown) for a particular gas hydrate are not the same. A drop in pressure or an increase in temperature will weaken the lattice of ice molecules encasing the gas molecules and allow the gas to liberate freely or disassociate and sublimate to gaseous state. Gas hydrates are a unique product because they may expand over one hundred times from their solid to gas form. This sublimation process can happen in the reservoir, the well bore, or on the surface.
Gas hydrates are an unstable resource due to their expansion characteristics when produced from a reservoir. Gas hydrate deposits have traditionally been treated only as a drilling hazard located in between the surface and a well's prime reservoir target deeper down. In addition, conventional drilling lacks the capacity to manage large quantities of a product that expands hundreds of times as it sublimates. This is unique to gas hydrates and an important issue for drilling and production.
Therefore, there exists a need in the art for a drilling system and method that is capable of drilling through long sections of a hydrates formation without substantially damaging the formation while controlling and handling disassociation of commercial quantities of gas hydrates.